The invention relates to an electrical circuit for generating a three-phase alternating current by means of a generator of low power output (10 kW to 5 MW), so that the current can be fed into a power grid. Generators with a small power output are particularly suitable as decentralized power supply units, also with co-generation of heat and power, as emergency power generators, as decentralized power plants for filling demand peaks in the power grid, or to supply power in certain areas that do not have a access to the grid of a public power utility.
Generators of low and intermediate power capacity of the kind described above are driven, for example, by gas turbines, fuel-cell/gas-turbine combinations, fermentation-gas engines, or diesel engines. These power generators have the advantage that they are able to meet variable power requirements. They can be regulated in response to the actual demand for electricity by controlling the amount of mechanical power produced by the drive sources of the generators. Thus, only as much energy is consumed as is needed to generate the required volume of electricity.
Wind-power plants, likewise, represent electricity producers with variable power delivery. Here, the parameters that determine the power input to the generator are represented by controllable quantities such as, e.g., the pitch angle of the rotor blades as well as non-controllable quantities such as wind velocity.
However, the aforementioned generators have the disadvantage that, as a rule, the power produced by them does not conform to the voltage and frequency requirements of the power grid to which the output terminals of the generators are connected. In particular, this is a problem with a demand-dependent regulation of the generator. Most of the aforementioned drive sources for low-power generators run at higher speeds and thus produce frequencies in the kilohertz range in a generator without a speed-reducing gearbox. The power grid, on the other hand, requires a frequency of 50 Hz or 60 Hz. A regulation that dynamically adapts to different requirements is particularly economical but causes a variable voltage of the output current in synchronous generators with permanent excitation. In synchronous generators with external excitation, the voltage can be adjusted by way of the excitation current of the generator, but this reduces the efficiency of the generator. A number of different arrangements of power-electronics circuits are known which work as inverters to perform the task of adapting the electrical quantities.
It may further be necessary to start the generator by using energy from the power grid or, if no power grid is available, from an external energy-storage device. In the latter case, it is practical to recharge the energy-storage device after the generator has been started.
A circuit for generators with permanent excitation or external excitation is known from U.S. Pat. No. 6,020,713, where the generator output voltage is converted to the intermediate-circuit DC voltage by means of two diode rectifiers. From the intermediate-circuit DC voltage, an output inverter generates an output voltage, which is then converted by a transformer into the voltage required by the power grid. This arrangement allows the generation of three-phase output voltages of 380V, 400V, 440V, 480V or 500V at frequencies of 50 Hz or 60 Hz. A three-phase power grid also requires a neutral connection. The arrangement just described has the disadvantage that it requires a transformer at the output side that is designed for the maximum possible output power level, which increases the cost of the power plant and takes up additional space.
The foregoing circuit arrangement according to U.S. Pat. No. 6,020,713 can also be used without the transformer at the output side, however in this case the maximum output voltage is limited to the level of the generator voltage. It is a disadvantage of this arrangement as well as of the previously described circuit that the start-up process of the power plant cannot be accomplished by simple means.
A further circuit arrangement proposed in U.S. Pat. No. 6,020,713 includes a fourth half-bridge for the active regulation of the neutral output connection, sharing the same drawbacks that were mentioned above.
Further proposed in U.S. Pat. No. 6,020,713 as well as in U.S. Pat. No. 6,093,975 are circuit arrangements that allow the generator to be used as starter motor for the gas turbine that drives the generator in its regular operating mode as a power plant. This is accomplished by adding a rectifier that is connected to the grid by means of a group of switches. At the same time the output of the inverter is connected to the generator through a group of switches and disconnected from the power grid by means of a further group of switches. The added rectifier feeds the intermediate circuit stage from the power grid. The inverter following the intermediate circuit stage powers the generator, which in this case works as a motor to start the gas turbine. In U.S. Pat. No. 6,093,975, a further circuit arrangement is proposed to supply the energy for the start-up of the gas turbine from an energy storage device such as a battery. This is accomplished through an external booster circuit to obtain a higher voltage from the battery to feed the intermediate circuit stage from which the generator (working as a motor) is powered to start the gas turbine.
Design solutions using an external transformer with either of the circuit arrangements just described have the known inherent drawbacks. A solution without the transformer, on the other hand, has the disadvantage that the maximum output voltage is tied directly to the generator voltage. As a further disadvantage, three groups of mechanical switches are required to switch the power plant to the start-up mode. To enable a battery-powered start-up, an additional circuit has to be provided.
The present invention therefore has the objective to provide a circuit for connecting a generator that is driven at a variable rpm-rate by a machine or wind turbine to a three-phase, 50 Hz- or 60 Hz power grid. Specifically, the objective calls for a circuit that can be realized with the smallest possible number of components and enables the generator to feed the power grid with a high degree of efficiency, independent of the generator rpm-rate or the generator voltage. The objective for the circuit according to the invention further specifies that the circuit must be able to tolerate an asymmetric load in the power grid, that it can be coupled directly to the power grid without an interposed transformer, and that it provides the capability to use the generator as a starter motor for the engine that normally drives the generator, with the energy for the start-up mode being supplied either by the power grid or by an external energy-storage device.
To meet the foregoing objective, the invention proposes a circuit for connecting a three-phase generator that is driven at a variable speed to a three-phase power grid. To support a forward operating mode where the energy flows from the generator to the power grid or to an energy-storage device, the circuit has a diode rectifier in combination with booster circuits on the side of the circuit that is nearest to the generator (which will also be referred to as the generator-adjacent circuit stage), an intermediate circuit stage formed by a series arrangement of two capacitor groups, and an inverter on the side of the circuit that is nearest to the power grid (which will also be referred to as the grid-adjacent circuit stage). To support a reverse operating mode where the energy flows from the energy-storage device or from the power grid to the generator, the circuit has a diode rectifier in combination with booster circuits in the grid-adjacent circuit stage, the aforementioned series arrangement of the two capacitor groups in the intermediate circuit stage, and an inverter in the generator-adjacent circuit stage. The entire arrangement of diode rectifiers, booster circuits and inverters together is formed by three generator-adjacent inductive elements, three grid-adjacent inductive elements, six generator-adjacent transistors and six generator-adjacent rectifier diodes, six grid-adjacent transistors and six grid-adjacent rectifier diodes. The neutral conductor of the power-grid is connected to the central junction of the three phase-associated windings of the generator and to the midpoint between the two capacitor groups.
In short, the circuit according to the invention consists of a symmetric arrangement of two three-phase inverters, an intermediate circuit stage of two capacitor groups arranged in series, inductance coils arranged after the generator, and a neutral conductor that is connected on the one hand to the center point of the star-like arrangement of the generator windings associated with the three phases and on the other hand to the midpoint of the serial arrangement of the two capacitor groups.
In the normal operation (also referred to as forward mode) of the circuit arrangement, where the flow of energy is directed from the generator to the power gridxe2x80x94and if the generator voltage after rectification to produce the intermediate circuit voltage is sufficient to feed the power grid directly through a subsequent inverterxe2x80x94the diodes of the inverter on the generator side function as an input rectifier that converts the AC voltage of the generator into the DC voltage of the intermediate circuit stage. The inverter that is arranged on the grid side of the circuit converts the intermediate-circuit voltage into the desired grid voltage with the appropriate frequency and feeds it through inductance coils, i.e., without a transformer, directly into the power grid.
Further in the normal operating mode of the circuit, if the generator voltage in the sense of the foregoing definition is too low, e.g., because of a low demand for energy and a corresponding low rpm-rate of the generator, the inverter and the inductance coils on the generator side work together as a booster circuit. Thus the intermediate circuit stage can be charged to the required DC voltage level, which the inverter on the grid-adjacent side converts into the desired grid voltage with the appropriate grid frequency, to feed it again (as in the preceding case) directly through inductance coils into the power grid.
In the starter mode, where the flow of energy is directed into the generator and where the generator works as a motor to start the actual drive source (e.g., a gas turbine), one has to differentiate again between two possibilities.
If the power for the start-up operation is supplied from the power grid, the diodes of the inverter in the grid-adjacent circuit stage work as an input diode bridge to generate an intermediate circuit voltage from the grid voltage. The intermediate circuit voltage is converted by the inverter in the generator-adjacent circuit stage into an AC voltage to drive the generator which in this case operates as a motor.
If there is no power-grid voltage available, an energy-storage device is connected to the circuit in an arrangement where the inverter on the grid side of the circuit is disconnected from the power grid by means of a group of switches and where two of the three output terminals of the inverter are connected to the energy-storage device by means of a further group of switches. To start the power plant with energy from a storage device, the inverter on the grid side of the circuit is disconnected from the grid and connected instead to the energy-storage device. Since typical energy-storage devices have a lower voltage than is necessary to operate the generator as a motor, the inverter and the inductance coils on the grid side of the circuit work together as a booster circuit and thus generate the required voltage level in the intermediate circuit stage, so that the generator can be operated as a motor by means of the inverter on the generator-adjacent side of the circuit.